Historically, to obtain center-distance measurement between holes, or from an edge to the center of a hole, was to zero-in the top and the bottom of the hole to obtain the diameter of the hole. Since the center is the midpoint of the diameter, the calculation is divided by two to get the true center in one axis. To measure in the other axis the workpiece is flipped over 90 degrees, and the operation is repeated.
Efforts to eliminte the above calculations in locating the center of a hole have resulted in the use of contact members in the form of "wedges" or cones. However, these members must enter the hole squarely to get an accurate reading, an almost impossible feat to accomplish with the unaided eye, resulting in one edge being closer to the center than the other.
A sphere is the only geometric shape which has all points on its surface equidistant to the center. Balls have been used in the past to locate the center of holes, particularly the center of the outer edge of chamferred holes, but because of friction in non-floating height gages, this method does not work satisfactorily. However, the recent introduction of the automatic reversing dial test indicator, which is almost frictionless, makes it easy to zero-in on the center in one axis if this axis is at zero, then with a ball, the other axis is at zero, and the workpiece needs only to be flipped over to measure its location in the other plane.
This friction in the sliding, and consequently the measuring feature of these gages has given rise to the use of air-bearings, both in height gages and in measuring machines. Air-bearing, or "frictionless" measuring devices do use ball-probes. Added to this, there is always the strong possibility of interjecting errors when points of reference are changed, or when probes are changed, not to mention the loss in time in making probe changes.
Great care must be exercised in building air-bearing measuring devices. Not only must tight tolerances be specified, but also flatness and squareness, and as a result these measuring devices are quite expensive. Air-bearing height gages require a "squareing fixture" when mounting "wedges" or cones. This fixture alone is as expensive as the best automatic reversing dial test indicator, which most shops already have.